As Rep. Tierney continued to deny any knowledge that his brothers-in-law were involved in illegal activity, some constituents were skeptical and insisted that he must have known more than he is letting on.

SALEM — Here in John F. Tierney’s hometown, many voters call the congressman by his first name, a familiarity born from years of political support.

But as the eight-term ­incumbent faces persistent questions over whether he knew about his in-laws’ illegal gambling operation, even long-time backers say they now have their doubts.

“John’s a friend of mine, and we go back a long time,” said David Hallinan, a Salem lawyer who said he has voted for Tierney throughout his congressional career. “But this has shaken my support a little. Whether this pushes me over the edge or not remains to be seen.”

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As he continued to deny any knowledge that his brothers-in-law were involved in illegal activity, some constituents were skeptical and insisted that Tierney must have known more than he is letting on.

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“Bulger didn’t know where Whitey was either,” John ­MacLean said, referring to William Bulger, the former politician and brother of gangster James “Whitey” Bulger. “Let’s get real. At Thanksgiving and Christmas, people talk.”

While the pervasiveness — and political impact — of such doubts is hard to gauge, some voters in the Sixth Congressional District said Tuesday that they are less likely to vote for Tierney than they were before.

MacLean, a 49-year-old from Lynn, said he had voted for Tierney as long as he could remember, but would find it difficult to throw his support behind someone he believes is lying.

At a press conference Tuesday, Tierney said he was aware his wife’s brothers, Robert and Daniel Eremian, ran a gambling operation but thought it was legitimate. Both brothers said last week that Tierney had lied about what he knew.

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Tierney has not been ­accused of criminal wrongdoing. But his wife, Patrice, pleaded guilty in October 2010 to tax fraud and “willful blindness” of the gambling operation.

Some voters said they suspect that John Tierney is guilty of willful blindness, too.

“How could you not know?” asked Marcia Guess, a 73-year-old from Beverly. “It doesn’t add up, and it’s not kosher.”

Guess, who described herself as a conservative Republican, said she hoped doubts over Tierney’s credibility would boost Richard R. Tisei, a former state senator who is challenging Tierney.

Some supporters remained staunch. Blythe Purdin, who referred to the congressman by his given name, said she ­believed Tierney, and would continue to support him.

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“I think that he’s telling the truth — he didn’t know,” said the 30-year-old from Salem.

At the same time, Purdin said Tisei appears to be a strong challenger, and that questions around Tierney were bound to help the Republican’s cause.

“This is definitely going to be a challenge for John,” she said. “But this seat is crucial, especially right now with what’s going on in Washington.”

The timing of the controversy, well before people typically turn their attention to political campaigns, may prove beneficial to Tierney, voters said. Several said they had not followed the recent accusations against Tierney, or were only broadly aware of the developments. Others shrugged off the news as trivial, just ­another example of the negativity that surrounds campaigns. But some felt offended by Tierney’s denials, and said his explanation was farfetched.

“The fact that he didn’t know what’s going on with his wife, what does that say about him?” asked John Morris, a lawyer from Saugus. “I’m not sure I can have faith in someone who doesn’t know what’s going on in their personal life.”

Morris said he had voted for Tierney before but is undecided this time around.

Even voters who were largely unaware of the controversy said they planned to read up on the details to make up their own mind. In Salem, one college student said she was looking forward to voting for the first time. Her hope was to find a candidate she felt she could trust.

“Integrity’s a big part of who I’ll vote for,” she said.

Peter Schworm can be reached at schworm@globe.com. Follow him on Twitter @globepete.